The disclosure of Japanese Patent Application No. 2000-082748 filed on Mar. 23, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to electric energy charging control apparatus and method for a hybrid vehicle. More specifically, the invention relates to electric energy charging control apparatus and method for hybrid vehicle, that achieve efficient utilization of an electric energy storage device while allowing a size reduction of the electric energy storage device in a hybrid vehicle that needs a large amount of electric energy output in order to assist the running of the vehicle through the use of a motor-generator.
2. Description of the Related Art
Vehicles equipped with hybrid vehicle (HV) systems that achieve great advantages in environmental protection and fuel economy improvement (hereinafter, referred to as xe2x80x9chybrid vehicles (HV)xe2x80x9d) are being developed and commercialized. An HV system is a power train that uses a combination of two kinds of drive power sources, for example, an internal combustion engine (a gasoline engine, a diesel engine, etc.) and an electric motor. By selectively using the engine and the electric motor in accordance with the driving condition, the system makes full use of the advantages of the two drive power sources, and supplements disadvantageous aspects of the two drive power sources with each other, so as to achieve smooth and highly responsive power performance. That is, by operating one of the engine and the electric motor alone or both of them in concert, the system is able to improve fuel economy and considerably reduce exhaust emissions. For example, during a low-load region where the engine efficiency is low (in particular, at the time of a vehicle start or a very low vehicle speed), the engine is not started, but the electric motor alone is operated to drive the vehicle. When the vehicle enters a speed region where the engine efficiency is high, the engine is started and the electric motor is stopped. When an increased output is needed, for example, during acceleration or the like, the engine and the electric motor are simultaneously operated to perform torque assist using the electric motor so that a desired output can be obtained.
When the electric motor is used in this manner, electric power is supplied from a battery installed in the vehicle. Therefore, the hybrid vehicle needs to be equipped with a large-capacity battery. In order to realize good use of the electric motor as described above, the state of charge (SOC) of the battery must always be controlled.
A typical hybrid vehicle is equipped with a motor-generator (MG) that performs an electric motor function and a power generating function. The MG is controlled so as to generate electric power so that the amount of charge in the battery converges to a target value of charge of the battery (target SOC). For example, Japanese Patent Application Laid-Open No. HEI 11-299004 discloses a control method for maintaining a targeted SOC by adjusting the engine output in accordance with the SOC.
Normally, the target SOC of a hybrid vehicle is set to a fixed value (e.g., an amount of charge being 60% of the full amount) with such a good margin between an upper limit and a lower limit that a discharge request (an electric motor drive request) and a charge request (a request for power charging through regeneration) can be accepted.
However, with regard to the hybrid vehicles, there are demands for reductions in vehicle weight, increases in compartment space, reductions in vehicle cost, etc. Therefore, battery size reductions are needed. If a battery is reduced in size, the battery capacity naturally reduces. In that case, therefore, a problem arises when the battery is to be charged or discharged. That is, the amount of charge or discharge allowed with reference to the target SOC reduces. As a result, it becomes impossible to discharge an amount that is needed at the time of a vehicle start or acceleration. A problem also arises at the time of deceleration. That is, only a small amount of energy can be charged into the battery although a large amount of regenerative electric power is generated. Thus, the amount of electric energy generated cannot be sufficiently utilized, and efficient utilization of energy (battery) cannot be realized.
Furthermore, the chemical reactions that occur inside batteries become slow when the battery ambient temperature decreases. Therefore, at low temperatures, the charging/discharging efficiency decreases, and sufficient charging/discharging becomes impossible even when the state of charge has converged to a target SOC. Therefore, according to the conventional art, it is inevitable to provide large-capacity (large-size) batteries in preparation for low ambient temperatures. Thus, the conventional art cannot meet the demand for a battery size reduction.
Accordingly, it is an object of the invention to provide an electric energy charging control apparatus of a hybrid vehicle that is capable of performing the requested charging/discharging at a high efficiency while allowing a size reduction of an electric energy storage device.
In accordance with a first aspect of the invention, an electric energy charging control apparatus of a hybrid vehicle includes an internal combustion engine, a motor-generator capable of assisting a run of the vehicle, an electric energy storage device connected to the motor-generator, controller that predicts a future state of charge/discharge of the electric energy storage device and changes a target value of charge of the electric energy storage device based on a result of prediction regarding charge/discharge of the electric energy storage device.
According to this construction, if it is predicted that the electric energy storage device will be discharged in the future, the target amount of charge of the electric energy storage device is changed to an increased value to increase the amount of charge beforehand, so that when the discharging occurs, an increased amount of discharge from the electric energy storage device can be provided. Conversely, if it is predicted that the electric energy storage device will be charged in the future, the target amount of charge of the electric energy storage device can be changed to a reduced value to reduce the amount of charge beforehand, so that when the charging occurs, an increased amount of charge into the electric energy storage device can be achieved. Therefore, a substantial electric energy charging/discharging range can be expanded. As a result, it becomes possible to efficiently perform charging/discharging as requested while allowing a size reduction of the electric energy storage device.
In the above-described aspect, the controller may predict the future state of the charge/discharge of the electric energy storage device based on a state of the run of the vehicle, and may increase the target value of charge when the state of the run of the vehicle is a state where it is predicted that at least a predetermined amount is discharged from the electric energy storage device, and the target value changing means may reduce the target value of charge when the state of the run of the vehicle is a state where it is predicted that at least a predetermined amount will be charged into the electric energy storage device.
The controller performs prediction regarding the charge/discharge of the electric energy storage device based on, for example, vehicle speed information. For example, if a low vehicle speed continues for a predetermined time, it is predicted that the vehicle will be stopped or greatly accelerated in the future. In association with a stop or a great acceleration, a large amount of electric energy will be consumed by the electric motor function of the motor-generator. Therefore, the target amount of charge of the electric energy storage device is increased to secure a sufficient amount of charge beforehand. Conversely, if a high vehicle speed continues for a predetermined time, it is predicted that the vehicle will be decelerated in the future. At the time of a deceleration, a great amount of regenerative energy will be obtained by the power generating function of the motor-generator. Therefore, the target amount of charge is reduced to increase the region for recovery of regenerative energy beforehand, so that regenerative energy will be sufficiently recovered. This construction makes it possible to efficiently perform charging/discharging as requested while allowing a size reduction of the electric energy storage device.
In the above-described aspect, the controller may change the target value of charge of the electric energy storage device in accordance with a vehicle ambient temperature.
According to this construction, if the vehicle ambient temperature is low, for example, below the freezing point, the target value of charge is increased so as to compensate for a reduction in the charging/discharging efficiency of the electric energy storage device caused by low temperature. Therefore, it is possible to efficiently perform charging/discharging as requested while allowing a size reduction of the electric energy storage device.